This summer I went to Sardinia, we did a route in the North of the island, for 6 days check out my route map below, points in red are where we stopped to sleep. Sardinia is a fantastic island where you can enjoy good weather, very hot temperatures, around 40º in July, they have an amazing gastronomy (fish, great meat, pasta and pizza of course, ice creams...) and wonderful landscapes.
We started out trip landing in Alghero airport in Fertilia where we rent a car during our days in the island.Regarding transport the car is the best option in the island to go around as the public transport it is in local buses. The train has only two routes in the island: Olbia-Sassari and Porto Torres-Cagliari.
In Alghero we visited the city center, there were very beautiful narrow streets and shops full of coral and turquoise jewelry. We also visited the area around the coast and the beaches around.
A good thing to do is to go to Capo di Caccia that is near Alghero and visit la Grotta de Nettuno that is a famous cave accessible from the sea by boat. There are plenty of day trips to the cave and also to go around the area in a boat and are around 10 euros.
The Grotta di Nettuno can be accessed as well by car, there is a small park car in the cliffs where you can take very good pictures. You can leave the vehicle at the side of the road near to the start of the steps to walk down to the cave.
There are 656 steps that lead to the cave and this descent is an experience itself. These steps are called the Escala del Cabirol or goat's steps, they were cut in 1954 and zizag down the cliff before going horizontally to the entrance of the Grotta.
Save another time to go to Stintino, to Pelosa and Pelosetta beach, this are the most famous ones, but you will find as well little beaches around some with rocks and some others with white sand that are less crowded.
Here you have a video about Sardenya I found on youtube very accurate photos of what I have seen: an amazing land, white sand beaches, beautiful towns and landscapes.